The present invention generally relates to so-called high electron mobility transistor (HEMT) devices and more particularly to a HEMT device having a heterojunction between an indium gallium arsenide (InGaAs) active layer and an n-type aluminium indium arsenide (AlInAs) electron supply layer and a manufacturing method thereof.
A direct-coupled FET logic (DCFL) device comprises an enhancement-mode field effect transistor (FET) and a depletion-mode FET and is characterized by low power consumption. Thus, the device is suited for constructing a high speed integrated circuit having a large integration density. In relation to the DCFL device, techniques have been studied intensively for manufacturing an inverter circuit for the DCFL device by employing a compound semiconductor device so as to improve the operational speed of the device further.
Conventionally, there is a known DCFL device comprising an enhancement-mode FET and a depletion mode FET both formed on a common semi-insulating gallium arsenide (GaAs) substrate as is disclosed in the U.S. Pat. Nos. 4,635,343 and 4,733,283, in which the assignee is the same assignee of the present invention. In this device, a two-dimensional electron gas is formed at a heterojunction interface between an undoped GaAs layer on the GaAs substrate and an n-type AlGaAs layer making a direct contact therewith. The two-dimensional electron gas is formed at an upper portion of the undoped GaAs layer and the electrons therein can move without experiencing scattering by the dopants. In other words, the electron mobility in the two-dimensional electron gas is increased and the operational speed of the device is significantly improved.
When manufacturing such a device, a layered body comprising the foregoing GaAs substrate, the undoped GaAs layer, and the n-type AlGaAs layer as well as an n-type GaAs layer provided on the n-type AlGaAS layer, a second n-type AlGaAs layer further provided on the n-type GaAs substrate, and a cap layer of n-type GaAs further provided on the n-type AlGaAs layer, is prepared. Further, a source electrode and a drain electrode are provided on the cap layer according to a predetermined pattern. Next, the layered body is applied with a photoresist and after a suitable patterning for exposing a part of the structure corresponding to a gate electrode of the enhancement-mode FET, the cap layer is removed by a dry etching procedure using a chloride etching gas. When the second n-type AlGaAs layer is exposed, the etching is automatically stopped because of the reduced etching rate in the AlGaAs layer. Note that the etching rate of AlGaAs by chloride gas is smaller by a factor of 200 than the etching rate of GaAs. Then, the n-type AlGaAs layer is removed by a wet etching and after a suitable photolithographic process for exposing another part of the structure corresponding to a gate of the depletion-mode FET, the first n-type GaAs layer corresponding to the gate of the enhancement-mode FET and the cap layer of GaAs corresponding to the gate of the depletion-mode FET are removed by again applying the dry etching using the chloride etching gas. This second dry etching also stops automatically when the n-type AlGaAs layer immediately above the undoped GaAs layer is exposed and when the second n-type AlGaAs layer is exposed. Then the gate electrode for the enhancement-mode FET and the depletion-mode FET are provided and an invertor circuit forming the DCFL is obtained. According to this procedure, the etching is stopped exactly at a desired depth as a result of use of the undoped or doped AlGaAs layer, and the enhancement-mode FET and the depletion-mode FET are formed with an exactly controlled threshold voltage.
Meanwhile, it is known that the operational speed of the HEMT device would be further improved if one could use a heterojunction of n-type aluminium indium arsenide (AlInAs) and undoped indium gallium arsenide (InGaAs). By using an InGaAs layer as the layer for supporting the two-dimensional electron gas, one can increase the electron mobility under a low electrical field and can obtain a high electron velocity under a high electrical field, too. Further, the electron density in the two-dimensional electron gas is increased because of the increased potential barrier established at the heterojunction. Furthermore, the decrease of the electron mobility by electron transfer to the low mobility band is avoided because of characteristic band structure of InGaAs which exhibits a large energy difference between the L valley and the valley. Additionally, there is a further advantage such that InAlAs is substantially free from unwanted deep donors.
Unfortunately, there is no known dry etching technique effectively applicable to the system, such as AlInAs or InGaAs involving indium (In), for providing the gate structure of a HEMT device and thus the manufacturing of the HEMT device using this promising material combination has been extremely difficult even in the case that the device is a simple FET. The device such as inverter is out of question. For example, the dry etching using carbon dichloro-difluoride (CCl.sub.2 F.sub.2) as the etching gas is virtually ineffective on a material such as InGaAs containing indium. It is believed that the reason for this is the significantly reduced equilibrium vapor pressure of indium trichloride (InCl.sub.3) which is formed as a product of the etching reaction (see S.C. McNevin, J. Vac. Sci. Technol., B4 (5) 1986, pp. 1216).
Because of the foregoing reasons, there is so far no report announcing success in constructing a HEMT integrated circuit device in which a heterojunction of n-type AlInAs and InGaAs is used.